The amount of data traffic on wireless communications continues to increase at an almost exponential rate. For example, many cell phone users expect their cell phones to routinely handle both the ability to surf the Internet at any time and to stream movies-sometimes at the same time. Thus, new ways of further maximizing data throughput are continually discussed and often implemented in each new version of a standard.
One way to increase throughput (when there are multiple receivers) is superposition multiple access (it is also known by other names), which will be described more fully below. This multiple access method has recently increased its importance as it is under serious consideration by the standards organization 3rd Generation Partnership Project (3GPP) to be part of the next Long Term Evolution (LTE) release. See, e.g., Chairman's Notes, 3GPP RAN1 Meeting #80b, Belgrade (2014, Apr. 20). Within and without 3GPP, the particular implementation of superposition multiple access being developed for probable implementation is often called Multi-User Superposition Transmission (MUST), but it has various names and different types, including, and not limited to, Non-Orthogonal Multiple Access (NOMA), Semi-Orthogonal Multiple Access (SOMA), Rate-adaptive constellation Expansion Multiple Access (EMA), Downlink Multiple User (DL MU), etc. The present disclosure is not limited to any of the afore-mentioned technologies, but has wide applicability to any superposition communication technology. Any of these terms as used in this disclosure should be understood in their proper context and/or broadest scope.
In general, multiple access superposition refers to communicating to multiple users by linearly combining amplitude-weighted, encoded, and/or modulated messages. For example, FIG. 1 has Base Station (BS) 110 and two users (or User Equipments (UEs)), a near UE 120 and a far UE 130 (“near” and “far” referring to their relative distances from BS 110). Both the near UE 120 and the far UE 130 receive the same signal x, comprising symbol xN for the near UE 120 and symbol xF for far UE 130, which can be represented by Equation (1):x=√{square root over (αN)}xN+√{square root over (αF)}xF  (1)
where α generally refers to transmission power, and thus αN is the transmission power allocated to the near user signal and αF is the transmission power allocated to the far user, where αN+αF=1. Sometimes a refers more generally to the ratio of near user power to far user power, as shown in FIG. 2, which is discussed further below.
Speaking simplistically, near UE 120 decodes symbol xF for far UE 130 and uses it to cancel xF as interference, thereby decoding symbol xN intended for the near UE 120. One reiterative process for this type of cancellation is “Successive Interference Cancellation” or SIC. The far UE 130, on the other hand, simply decodes its own signal xF(although it is possible for the far user to also perform some form of signal cancellation to eliminate xN).
Generally herein, far user symbol xF corresponds to KF bits of data represented as (d0Fd1F . . . dKF−1F) and near user symbol xN corresponds to KN bits of data represented as (d0Nd1N . . . dKN−1N).
FIG. 2 shows an example of a “super-constellation” formed of a (QPSK, QPSK) modulation pair under MUST. “(QPSK, QPSK)” means that both the far and near UE signals are modulated by QPSK. FIG. 2 is the result of a direct symbol mapping (DSM) of QPSK using Equation (1) for both the near and far users, i.e., a 16-QAM super-constellation. Moreover, in FIG. 2, the constituent xF and xN symbols are separately Gray encoded.
Each of the four bit symbols in the 16-QAM super-constellation in FIG. 2 comprises two bits for the symbol intended for the far user and two bits of the symbol intended for the near user. More specifically, each four-bit symbol (b0, b1, b2, b3) comprises (b0, b1)=(d0Fd1F), the two bits for the far user, and (b2, b3)=(d0Nd1N), the two bits for the near user. Thus, the far user constellation is relatively coarse, because each quadrant represents only one symbol (for example, the upper right quadrant is (00)), while each quadrant of the near user constellation has all four symbols (00, 01, 10, and 11). However, because the near user is nearer, the near user's received signal is stronger and it will be easier for the near user to distinguish that level of detail than the far user.
In theory, having the near user employ Successive Interference Cancellation (SIC) by codeword, where the far user codeword is decoded, the original encoded far user codeword reconstructed using the decoded codeword, and then the reconstructed original signal cancelled from the overall signal prior to decoding, is optimal in the sense that it achieves capacity.